This invention relates to a planar (two-dimensional) thermal head in which the thermal dots are arranged in a matrix and a display device incorporating the planar thermal head.
FIG. 1 is a cross-sectional view showing a prior art planar thermal head. As illustrated, it comprises a base 1 made of a material such as a ceramic. Running parallel to the X axis (left to right in the drawing) on the base 1 are a plurality of equally-spaced X wiring lines 2, and over these a thermally insulating layer 3 consisting of polyimide or another material with thermal insulating properties is applied. A plurality of resistive elements 4 are formed over this thermally insulating layer 3 to act as the heat-generating bodies. One side of each resistive element 4 is connected to the X wiring via a through-hole conductor 5. The other side is connected to one of Y wiring lines 6 which are arranged on the thermally insulating layer 3 at equally-spacd intervals running parallel to the Y axis direction (normal to the page). The resistive elements 4 thus form a matrix of thermal dots in the X and Y directions.
A thermal display device employing a planar (two-dimensional) thermal head similar to that shown in FIG. 1 is described in Japanese Laid-Open Patent Application No. 208787/1985. FIG. 2 shows a cross-sectional view of this planar thermal display, which comprises a glass substrate 11, a plurality of X wiring lines 12, a thermally insulating layer 13 of a material such as polyimide, a plurality of through-hole conductors 15, a plurality of Y wiring lines 16, a heat-sensitive, temperature-indicating layer 17, and a plurality of transparent resistive elements 14. To display an image, a single transparent resistive element 14 is selected by selecting one of the X wiring lines 12 and one of the Y wiring lines 16. Specifically, a voltage of 0 is applied to the selected terminal of the X wiring lines 12 and a voltage of 2/3E to the nonselected terminals, and a voltage of E is applied to the selected terminal of the Y wiring lines 16 and a voltage of 1/3E to the nonselected terminals. Thus a voltage of E is applied across the selected transparent resistive element 14 while a voltage of 1/3E is applied across the nonselected transparent resistive elements 14. As a result of receiving three times the voltage of the other electrodes, the transparent resistive element 14 selected according to the image data generates nine times as much heat. This local heating induces a color change in the temperature-indicating layer 17.
In the prior art planar thermal heads having the structure shown in FIG. 1, the following problems occur:
(a) The structure and fabrication process are complex, due to the need to provide one through-hole conductor 5 passing through the thermally insulating layer 3 and one resistive element 4 for each thermal dot.
(b) As noted in the Japanese Laid-Open Patent Application cited above, not all of the heat generated by the resistive elements 4 is conducted to the temperature-indicating layer. A substantial amount of the heat diffuses to the thermally insulating layer 3. This planar thermal head therefore requires a very large driving power.
The planar thermal display with the structure shown in FIG. 2 suffers from the problem of complex structure and difficult fabrication due to the need to connect the X and Y lines electrically by through-hole conductors passing through the temperature-indicating layer 17. Further complications of structure and fabrication arise from the need to provide the same number of transparent resistive elements as the dots. The cost of these devices is accordingly high.